|Simplicity. Simplicity can sometimes be an asset when under other time constraints.
Number of variables measured. Do I need more than precipitation and temperature? Multiple sensors may require a larger, more expensive datalogger.
Degree of automation. How often can I afford to take observations?
The method of telemetry. How do you plan to retrieve the data? Options include everything from a manual read of the sensor recorded on paper to radio, telephone, direct RS232 link to a personal computer, or even satellite.
In general, the greater the complexity and sophistication of the system, the greater the cost. Similar to tools and other purchases, there is a rule-of-thumb that applies here: "You get what you pay for." Remember this especially if you are looking for a system that needs to be working properly during many seasons of use.
Other options beyond buying
For some users, there are a couple of other options available besides purchasing your own equipment. Many states and some private companies have networks of automated weather stations that collect the information needed for IPM. If you are fortunate enough to be located near one of these stations, this may be a cost-effective option. Lastly, some companies are now offering localized weather information derived from larger-scale meteorological and climatological data. These so-called "off-site" data may be a less expensive and adequate alternative for some users, depending on the sophistication of the application. For most applications, however, data taken on your farm is still the best strategy.
As you prepare to put your new weather equipment in the field, there are some important issues to consider. The first is where to place the instruments. You likely only have one set of instruments, so it is therefore critical to place them in a location that will represent your orchard environment as closely as possible. In general, greatest spatial variability in a given region will be observed with leaf wetness, followed by precipitation, relative humidity, and air temperature. Thus, if you have the resources for extra sensors, the best investment would probably be in additional leaf wetness sensors or precipitation gages.
In deciding where to place your instruments, a rule-of-thumb is that the station should be at least four times the height of the closest obstruction and away from any large paved areas. Keep grass and weeds near the station trimmed. Also consider the effects of microclimate. Microclimate refers to time-averaged weather conditions over distances of less than half a mile, and includes climate effects from the orchard level down to the microscopic level. Endless combinations of characteristics such as crop canopy structure, topography, and soil type can create unique microclimates. Solar heating and airflow are the most significant factors determining the microclimate in a particular area. For example, locations within or very close to vegetation acting as a windbreak tend to be warmer and more humid than locations in the open. Low-lying basin-shaped areas tend to be prone to cold temperatures during the growing season since cold air ponds there after moving from surrounding areas. Besides serving as a location for a frost alarm for the first location to reach the freezing mark on a frosty night, these sites should generally be avoided for siting IPM instruments.
Try to select an area that is:
Relatively open with few obstructions to air flow in all directions;
Relatively flat, to minimize the impact of cold air drainage or forced turbulence around topographical obstacles;
Easily accessible to you night and day; and
Close enough to an orchard to place a leaf wetness sensor if one is available with your system.
In choosing a site to meet your needs, remember that it may be virtually impossible to meet all of these criteria. Try to minimize the number of overall potential problems.
Temperature and precipitation
The two weather variables most commonly monitored by growers are air temperature and precipitation. For precipitation, try to site your gage on level ground away from any overhead obstructions. Typical installation height is 3 to 4 feet above the ground. When installing on a post, make sure that the opening of the gage is above the top of the post. Keep the gage level, especially if using tipping bucket-type rain gages. When the gage is in use, periodically check for any cracks, insects, accumulated dust or bird droppings.
For placement of temperature sensors in the field, there is one special rule. Never allow the sensor to be directly exposed to the sun or open sky, as such exposure can lead to erroneous or misleading measurements. If at all possible, use a screened or ventilated shield or enclosure if one is available for this purpose. By convention, the sensor should be placed at a height 5 to 6 feet above the ground surface. Avoid installation near paved areas, on walls of buildings or other artificial heat sources, or near low places where water tends to pond after rain.
Relative humidity and leaf wetness
Relative humidity and leaf wetness measurements are of special importance, as one or both are typically needed for monitoring plant disease risk. For relative humidity, placement in the field is fairly easy. For many sensors and automated systems, the relative humidity probe is combined with a temperature sensor, and the entire probe is installed in a shielded enclosure. For leaf wetness, however, there are few set rules, as there are no agreed upon standards for measurement of the variable.
The first issue is what type of sensor to choose. Options range from a rope wick connected to a mechanical recorder to an electronic grid plate across which resistance can be measured. For simplicity and automation, many users choose an electronic grid. Some users prefer to have the sensor professionally coated with a latex paint. Paint causes the sensor to be physically more representative of a plant leaf and tends to spread out any water droplets over the surface, making the painted grid generally more sensitive to dew formation. For leaf wetting events associated with rainfall, painted and unpainted grids respond similarly. Placement of your leaf wetness sensor in the field may vary greatly depending on crop and disease. In contrast to other meteorological measurements that are taken in more open locations, the best bet with leaf wetness sensors is placement within the plant canopy. While there is no standard, some plant pathologists recommend placement in an apple canopy on the north side of the tree above the first scaffold about midway between the tree trunk and the drip line. Past research indicates a consistent response with the plate sloped at a 20-degree angle downwards towards the north.
Upkeep and maintenance
After your weather system is in the field, remember that upkeep and maintenance of the system and its sensors are essential for it to provide useful information for your IPM program. Plan to check sensors at least once and preferably twice per year. Follow the manufacturer’s guidelines closely for checking operation of the system and maintaining calibration. In general, the more sophisticated the sensor or device, the greater the need for periodic checks. For example, electronic relative humidity probes can lose calibration relatively quickly, and should be checked carefully at least once per year, and replaced if necessary. In contrast, manually read sensors such as liquid in glass thermometers and rain gages can provide years of service with minimal care. Many problems with sensors are spontaneous and easily detectable. Other problems, such as sensor drift, may be more difficult to detect without regular inspection or comparative analysis of the data from a second, nearby data source.
Some commercial sources of weather instruments and equipment
The accompanying table is a short list of some vendors who either manufacture or sell a variety of weather monitoring equipment. It is by no means complete, but provides a range of offerings regarding instrument accuracy, durability, and cost.